I love how Fireheart looks! Such a great idea! When you think about it...Scourge would fit in the same stance/emotional figure as Rusty/Firepaw/Fireheart/Firestar in your art! They both start off as a quiet kit kittypet and then get more determined and brave as they are about apprentice age, then they become fierce(despite the amount of difference between character and story they have the same development that can be seen in a drawing like this) as they become warrior age then a few moons later they are looking serious and strong as leaders.
I like the simple and normal design, you have picked for Firestar too. Like some other person said, it is a much better version than all the sparkle cats with stars and a strange hairstyle out there who should portrait him. I also like the expressions you have choose for the different stages. Since you can perfectly see where his thoughts and character has changed most during his way to become a leader and that he is not the same cat anymore when he takes over the command of the clan. The contrast between Rusty and Firestar is also depicted in the contrast between the orange fur of him and the pale background. I like the trees and bushes you have drawn there since they look like they are hidden in the early morning fog.
I don't know if you have read the books but I woul say that the different "stages" the cats there pass during their life ( -kit -paw, Warrior and then maybe even -star) could be seen as an evolution. Of course there are tons of other characters in the books, who have changed much more than Firestar did - also in their outer appearance - but since Firestar is the protagonist of the first Season and also my favourite character, I have drawn his evolution.
I think what the artist is trying to say or express is that this is evolution, or how Firestar changes through the entire series. True, it could use a better word, but evolution's definition is something that changes over time. [I might be wrong, but that's just my opinion.] o u o